This invention relates to golf balls and more particularly to an improved golf ball core useful in making two-piece balls having superior short iron and other playability characteristics.
For many years top grade golf balls have been made by molding balata, trans polyisoprene, trans polybutadiene, or various compositions including such elastomers about elastic, thread-wound cores. An experienced player can apply spin to a balata-covered wound ball such that it will fade or draw in flight or have the backspin necessary to stop abruptly on the green when hit with a short iron. These playability properties are most important in short iron play and can be exploited significantly only by relatively skilled players.
Balata and its synthetic substitutes today have essentially been replaced by new materials. With the exception of a few lines of golf balls distributed through pro shops to professional golfers and those who would emulate them, newer synthetic polymers are the cover materials of choice.
Of the new synthetics, by far the most commonly used are a line of ionomers sold by E. I. Dupont de Nemours & Company under the trademark SURLYN. These materials comprise copolymers of olefins, typically ethylene, with an alpha, beta, ethylinically unsaturated carboxylic acid such as methacrylic acid. Metal ions such as sodium or zinc are used to neutralize some portion of the acid groups in the copolymer resulting in a thermoplastic resin which has several advantages including a cost advantage over balata. The ionomers may be manufactured with a wide variety of properties which, in a golf ball cover, affect cut resistance, shear resistance, general durability, and resilience.
U.S. Pat. No. 3,819,768 to R. P. Molitor discloses that blends of sodium neutralized ionomer resins with zinc neutralized ionomer resins, as a class, have certain advantages which have not been achievable in any other way. Among these is the production of an unexpectedly high coefficient of restitution of golf balls having the blended ionomer cover. Such covers also resist cold cracking, have excellent aging properties, and are unexpectedly durable. The development of the SURLYN blended cover has been a major factor in the production of two-piece balls having covers which for all practical purposes cannot be cut in play, and which travel further when hit than any other USGA regulation ball as measured by controlled tests when hit by golfers or testing machines.
Such balls typically have a separately molded, solid resilient core. The core is manufactured by compression molding a slug of cross-linkable elastomer composition, e.g., those disclosed in U.S. Pat. Nos. 4,464,075, 4,169,599, 4,165,877, or 4,141,599. It is believed that most high quality two-piece balls sold in the United States currently have cores consisting of a high cis content polybutadiene, a zinc salt of an alpha, beta, ethylinically unsaturated monocarboxylic acid, e.g., zinc di or mono acrylate or methacrylate, and a small amount of zinc oxide, cured with conventional free radical initiator type catalysts, typically a peroxide.
While the balata covered, thread-wound balls are easily cut and very expensive, they nevertheless have a significant edge in short iron playability. It is much more difficult to impart spin to two-piece balls and thus more difficult to fade or draw drives or to chip with precision. Frequently, experienced players note that the ionomer covered two-piece balls, although long off the woods and irons, have an unsatisfactory "feel".
The manufacture of two-piece balls, i.e., balls comprising a solid, molded, resilient core and a cover, has many significant advantages over the more expensive wound balls. There is accordingly a need for two-piece balls having short iron playability characteristics comparable to wound, balata rubber-covered balls. A ball having such properties which also had the "distance" of ionomer blend covered two-piece balls would be attractive to golfers.